/Engine/Source/Runtime/GeometryCore/Public/Image/ImageBuilder.h
Very basic downsampling technqiue that just averages NxN pixel blocks. Multi-threaded.
[TImageBuilder](API\Runtime\GeometryCore\Image\TImageBuilder)< PixelType > FastDownsample ( int32 SubSteps, const PixelType & ZeroValue, [TFunctionRef](API\Runtime\Core\GenericPlatform\TFunctionRef)< PixelType(PixelType, int)> AverageFunc ) const